firmware: introduce sysfs driver for QEMU's fw_cfg device

Make fw_cfg entries of type "file" available via sysfs. Entries
are listed under /sys/firmware/qemu_fw_cfg/by_key, in folders
named after each entry's selector key. Filename, selector value,
and size read-only attributes are included for each entry. Also,
a "raw" attribute allows retrieval of the full binary content of
each entry.

The fw_cfg device can be instantiated automatically from ACPI or
the Device Tree, or manually by using a kernel module (or command
line) parameter, with a syntax outlined in the documentation file.

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>